"Rock of the Month # 291, posted for September 2025" ---

Precambrian Iron Formations

Banded iron formations (BIF) account for 95% of global iron ore resources (Gross, 1993). They are well distributed in wide areas of Canada and the USA, Australia, India, Brazil, Ukraine and elsewhere. These siliceous, iron-rich sediments formed by exhalation of hot fluids associated with sea floor volcanic activity, as seen in the modern submarine vents known as black smokers. They are a prominent form of chemical sediment. They are primarily of Precambrian age (Archean and early Proterozoic, circa 2700 to 1800 Ma). They are variably composed of oxide, silicate, carbonate and sulphide minerals (e.g., hematite and magnetite; quartz; siderite; pyrite). So-called Lake Superior BIF occur with black shales, quartzite and dolomite (an inferred continental shelf setting), whereas Algoma-type BIF (named for a region of central Ontario east from Lake Superior) include greywacke sediments and lavas, and are thus associated with volcanic centres.

Local geology and stratigraphy

The stratigraphy and mineral deposits of northeastern Minnesota, including slates, greywacke and conglomerate, lavas, porphyries and iron formation, has been documented in detail over the years (e.g., Gruner, 1941; Grout et al., 1951; Sims, 1972; Jirsa and Green, 2011). The Soudan mine area includes basaltic to andesitic lavas, pillow lavas and volcanic breccias, and intercalated chemical sediments, notably the Soudan BIF and associated rocks (Moosavi et al., 2008; Radakovich et al., 2011; Vallowe et al., 2011).

Soudan is in the Vermilion greenstone belt of the Wawa subprovince, rocks of late Archean age, circa 2700 Ma. The Soudan mine is in the Soudan belt, volcanic sequences and turbidites (to the north, the Newton belt includes volcanic and volcaniclastic rocks, and intrusives, with some komatiitic flows and peridotitic sills: Jirsa et al., 1992, 1993; Peterson et al., 2009)). The Soudan mine and its BIF outcrops also comprise a classic fieldtrip stop (e.g., Bauer et al., 2011). The Vermilion district is the best example of an Archean greenstone belt in the USA (Southwick, 1987).

In 1882, Soudan became the first Fe mine in Minnesota, working massive hematite ore. Mining commenced by open pits, but by 1892 all mining was underground. The mine is open to the public for educational trips and the mine is now a state park: out of 13 shafts at the mine, two remain. The No.8 shaft, still in use today for trips, reached a depth of 2,400 feet. Competition from taconite ores of the Mesabi Range led to mine closure at Soudan in 1962. The 27th level was initiated but never reached production due to the mine closure. Mine tours today go down to the 27th level and access the Montana orebody, the last body to be developed and mined here (Peterson et al., 2009).

Banded iron formations are a major source of sedimentary iron ores (e.g., Gross, 1991, 1993, 1995). Algoma-type BIF is often relatively enriched in trace elements, including gold and arsenic (Boyle, 1991; Gourerol et al., 2016). Iron formations are abundant around Lake Superior (e.g., Laberge, 1994).

A limited range of mineral species occur at the Soudan mine - the most prized specimens are clear quartz prisms, many of which are coloured red towards their terminations by hematite inclusions (Polytika, 2014, p.698; Wilson and Olson, 2017; Moore, 2017, p.201). Besides hematite and jasper (the principal ore) the mine has yielded the quartz crystals, some native copper, as well as some pyrite, carbonates (siderite, dolomite, rhodochrosite), chlorite and other minerals (Wilson and Olson, 2017).

Particle physics underground, most notably in old mines

Neutrino detectors and associated laboratory experiments (Fraser, 1998) occur in a number of deep sites around the world, including: Sudbury (Ontario, Canada); Homestake (S.Dakota, USA); Soudan (Minnesota, USA) and Kamiokande (Japan). These north American examples are all in old mine workings (for nickel, gold and iron, respectively).

The Soudan lab is one of a regional cluster of labs related to Fermilab (Jirsa and Miller, 2010). The Soudan underground laboratory of the University of Minnesota has operated here since 1980. It is 710 m (2,329 feet) underground, with equipment designed for detection of neutrinos and WIMPs (weakly interacting massive particles) generated at Fermilab, 735 km distant (Peterson et al., 2009). It was involved in gathering early evidence for a small but non-zero mass for neutrinos (Watson, 1997). Marvin Marshak (2009) gave the Institute on Lake Superior Geology an engaging and humourous banquet presentation on CERN, particle physics, and the research conducted at the lab in the Soudan mine. Apparently, in northern Minnesota, a person is struck every 2.2 seconds by a neutrino generated at Fermilab in Chicago. The MINOS detector in the mine is constructed of 5,500 tonnes of iron metal in 485 1" (25.4 mm) sheets. Muon neutrinos are made in Fermilab and beamed toward Minnesota: they enter the state near Two Harbors and are detected at Soudan: in 2009, a final detector was under construction at the NOVA facility at Ash River.

REFERENCES

Boyle,RW (1991) Auriferous Archean greenstone sedimentary belts. In `Historical Perspectives of Genetic Concepts and Case Histories of Famous Discoveries' (Hutchinson,RW and Grauch,RI editors), Econ.Geol. Monograph 8, 359pp., 164-191.

Fraser,G (editor) (1998) Neutrino monitor. CERN Courier 38 no.6, 1-11, September.

Gourcerol,B, Kontak,DJ, Thurston,PC and Duparc,Q (2016) Do magnetite layers in Algoma type banded iron formations (BIF) preserve their primary geochemical signature? A case study of samples from three Archean BIF hosted gold deposits. Can.Mineral. 54, 605-624.

Gross,GA (1991) Genetic concepts for iron formation and associated metalliferous sediments. In `Historical Perspectives of Genetic Concepts and Case Histories of Famous Discoveries' (Hutchinson,RW and Grauch,RI editors), Econ.Geol. Monograph 8, 359pp., 51-81.

Gross,GA (1993) Industrial and genetic models for iron ore in iron-formations. In `Mineral Deposit Modeling' (Kirkham,RV, Sinclair,WD, Thorpe,RI and Duke,JM editors), GAC Spec.Pap. 40, 770pp., 151-170.

Gross,GA (1995) Algoma type iron formation. In `Geology of Canadian Mineral Deposit Types' (Eckstrand,OR, Sinclair,WD and Thorpe,RI editors), GSC Geology of Canada, no. 8 / GSA Geology of North America vol. P 1, 640pp., 66-73.

Grout,FF, Gruner,JW, Schwartz,GM and Thiel,GA (1951) Precambrian stratigraphy of Minnesota. Bull.Geol.Soc.Amer. 62, 1017-1078.

Gruner,JW (1941) Structural geology of the Knife Lake area of northeastern Minnesota. BGSA 52, 1577-1642 plus map.

Jirsa,MA and Green,JC (2011) Classic Precambrian geology of northeast Minnesota. In `Archean to Anthropocene: Field Guides to the Geology of the Mid Continent of North America' (Miller,JD, Hudak,GJ, Wittkop,C and McLaughlin,PI, editors), GSA Field Guide 24, 544pp., 25-45.

Jirsa,MA and Miller,W (2010) Ash River neutrino detector laboratory (NOvA) and the Archean Vermilion granitic complex. Institute on Lake Superior Geology, volume 56 part 2, 142pp., trip 5, 79-80, International Falls, MN.

Jirsa,MA, Boerboom,T and McSwiggen,P (1993) Geology of Archean greenstone granite terrane in the Cook to Side Lake area. Institute on Lake Superior Geology, Proceedings Volume 39 part 2 - Field Trips, 158pp., 97-127, Eveleth, MN.

Jirsa,MA, Southwick,DL and Boerboom,TJ (1992) Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: refolding of precleavage nappes during D2 transpression. Can.J.Earth Sci. 29, 2146-2155.

LaBerge,GL (1994) Geology of the Lake Superior Region. Geoscience Press, Boulder, CO / reprinted by Penokean Press, Oshkosh, WI, 313pp.

Marshak,M (2009) The deep underground sky. Abs. 55th Annual Meeting, Institute on Lake Superior Geology, vol.55 part 1, 83pp., xxiv (see also www.soudan.umn.edu), Ely, MN.

Moore,TP (editor) (2017) Mineral Collections in California. Mineral.Record 48 no.4, supplement, 296pp.

Moosavi,S, Johnson,TK, Wendland,C, Anderson,A and Hudak,GJ (2008) Bedrock geology of the footwall to the Soudan iron formation south of Twin Lakes, St. Louis county, northeastern Minnesota. Abs. 54th Annual Meeting, Institute on Lake Superior Geology, vol.54 part 1, 80pp., 57-58, Marquette, MI.

Peterson,D, Pointer,J and Marshak,M (2009) Soudan iron mine and physics lab. Institute on Lake Superior Geology, volume 55 part 2, 215pp., trip 3, 100-109, Ely, MN.

Polytika,J (2014) Springfield show 2014. Mineral.Record 45, 693-698.

Radakovich,AL, Parent,CT, Partridge,ME, Ritts,AD, Pierce,R and Hudak,GJ (2011) Reconnaissance bedrock geological map of the northern part of Soudan Underground Mine state park and the northwestern part of Lake Vermilion state park, St. Louis county, Minnesota. Abs. 57th Annual Meeting, Institute on Lake Superior Geology, vol.57 part 1, 98pp., 67-68, Ashland, WI.

Sims,PK (1972) Mineral deposits in lower Precambrian rocks, northern Minnesota. In `Geology of Minnesota: a Centennial Volume' (Sims,PK and Morey,GB editors), Minnesota Geol.Surv., 632pp., 172-176.

Southwick,DL (1987) Geologic highlights of an Archean greenstone belt, western Vermilion district, northeastern Minnesota. GSA Centennial Field Guide Vol. 3, North Central Section (Biggs,DL editor), 448pp., 53-58.

Vallowe,AM, Thalhamer,EJ, Rhoades,DL and Peterson,DM (2011) Surface and subsurface geologic maps of the Soudan Underground Mine state park, St. Louis county, northeastern Minnesota. Abs. 57th Annual Meeting, Institute on Lake Superior Geology, vol.57 part 1, 98pp., 87-88, Ashland, WI.

Watson,A (1997) Case for neutrino mass gathers weight. Science 277, 30-31, 04 July.

Wilson,WE and Olson,DK (2017) The Soudan mine, Vermilion Range, St. Louis County, Minnesota. Mineral.Record 48 no.4, 493-524.